The present invention pertains to the production of a laminated web, such as corrugated paperboard and, more particularly, to a double backer in which a pre-heating element is used in conjunction with a standard heating element to form the adhesive bonds in the paperboard web.
In a typical prior art double backer, a liner web is brought into contact with the glued flute tips of a single face corrugated web, and the freshly glued double face web is then passed over the surfaces of a number of serially arranged heating units, usually steam chests, to cause the starch-based glue to set and to drive moisture from the web. Double face web travel over flat heated surfaces of steam chests is typically provided by a wide driven holddown belt in direct contact with the upper face of corrugated web. The top face of the belt, in turn, is held in contact with the traveling web by any of several types of weight or force applying devices, well known in prior art. For example, the holddown belt may be engaged by a series of weighted ballast rollers, it may be forced into contact with the web by air pressure from a system of nozzles located over the web, or an arrangement of inflatable air bladders may be operated to press the moving holddown belt into contact with the double face web. It is also known to provide means for varying the ballast load applied to the holddown belt and web, both longitudinally in the machine direction and laterally in the cross machine direction.
The use of a driven holddown belt in a double backer has a number of attendant disadvantages. The holddown belt must be mounted for continuous travel in the manner of the conventional continuous conveyor belt system and, therefore, must also include a separate belt drive means. The holddown belt also must necessarily overlie the entire surface of corrugated web, at least in the heating section, and, as a result, may inhibit the escape of moisture from the board as it dries. Also, the edges of the belt which overhang the edges of the corrugated web run in contact with surfaces of the steam chests or other heating surfaces and are subject to wear.
In a commonly owned, co-pending patent application Ser. No. 08/643,627, a double backer is provided in which the driven holddown belt is eliminated. Stationary holddown strips, extending parallel to one another in the direction of web movement, are supported from above to contact the entire web across its width and along the heating section. A separate downstream vacuum conveyor is used to pull the corrugated web through the heating section.
The double backer, as previously described, applies heat through the use of the serially arranged heating units only to the bottom side of the web as it is being constructed. While applying heat to only one side is sufficient in most cases, it has been found that the heat transfer from the single sided heating units is often insufficient to cure the additional glue bonds when the double backer is running heavy weight double or triple wall board. For instance, when running triple wall board, which consists of three layers of single face web bonded together with an outer liner, the heat from a conventional lower heating unit raises the temperature of the starch close to the heating units, gelling the starch and flashing the excess water to steam. The steam then migrates through the wall board, heating the more remote glue lines. Although this system is sufficient to cure the bonds between the lower outer liner and its adjacent fluted medium layer, the conventional lower heating units do not transmit sufficient heat to the more remote bonds which can cause problems to occur, including inadequate bonding and insufficient drying. To solve this problem, the double backer, and thus the entire corrugator, must be slowed considerably to allow for adequate heating.